Technical Data
MC68HC11E Family
—
Rev. 4
210
Analog-to-Digital (A/D) Converter
MOTOROLA
Analog-to-Digital (A/D) Converter
10.3 Overview
The A/D system is an 8-channel, 8-bit, multiplexed-input converter. The
converter does not require external sample and hold circuits because of
the type of charge redistribution technique used. A/D converter timing
can be synchronized to the system E clock or to an internal resistor
capacitor (RC) oscillator.
The A/D converter system consists of four functional blocks: multiplexer,
analog converter, digital control, and result storage. Refer to
Figure 10-1
.
10.3.1 Multiplexer
The multiplexer selects one of 16 inputs for conversion. Input selection
is controlled by the value of bits CD:CA in the ADCTL register. The eight
port E pins are fixed-direction analog inputs to the multiplexer, and
additional internal analog signal lines are routed to it.
Port E pins also can be used as digital inputs. Digital reads of port E pins
are not recommended during the sample portion of an A/D conversion
cycle, when the gate signal to the N-channel input gate is on. Because
no P-channel devices are directly connected to either input pins or
reference voltage pins, voltages above V
DD
do not cause a latchup
problem, although current should be limited according to maximum
ratings. Refer to
Figure 10-2
, which is a functional diagram of an input
pin.